AX5043 Receiver Configuration

Hello,

I am trying to tune the AX5043 reception chain but I am getting some weird behavior on the Automatic Frequency Control loop.

In my setup, I am transmitting a GSMK-modulated signal (0101 data) at 2.4kbps to the input of the transceiver, while monitoring the value of the TRKRFFREQ register. I have calculated the value of the MAXRFOFFSET to allow a maximum frequency deviation of ± 600 Hz (bitrate / 4). I am also monitoring the decoded bitstream output using the DATA and DCLK pins of the transceiver with a scope. For this test, I am using only RXPARAMSET 0.

The AFC seems to lock to the incoming signal easily when the transceiver is initialized and configured with the RF signal already at its input. When I vary the frequency of the carrier, I can also see the TRKRFFREQ varying accordingly.

However, when the transceiver is configured and initialized without any RF signal at its input, the TRKRFFREQ goes to its maximum value (~600) and doesn’t really move anymore. When I transmit the modulated signal again, it is not able to track it anymore. The only way I can get the receiver to work again is by sweeping the carrier frequency and forcing (hoping) that the AFC will be able to lock again.

Based on what I read, registers FREQUENCYGAINC and FREQUENCYGAIND can be used to tune the gain of the AFC loop, but I haven’t found specific information about how to calculate them.

I tried to change the values of these registers, and I can clearly see that they have a lot of impact on the AFC, but I am not sure exactly how. Could you help me shed some light on how to compute these registers and how they impact the behavior of the AFC? Also, is there any other possible cause for this problem?

Any help would be appreciated.

Thanks in advance